Organic light emitting diodes are self-emission devices. Organic light emitting diodes have wide viewing angles, high contrast ratios, quick response speeds, excellent luminescent characteristics, low driving voltages, and excellent colorizing characteristics.
In general, an organic light emitting diode includes a substrate, an anode disposed on the substrate, a hole transport layer, an emission layer, an electron transport layer and a cathode, which are sequentially disposed on the anode in this order. Herein, the hole transport layer, the emission layer and the electron transport layer are organic thin films.
An operational principle of the organic light emitting diode will now be described in detail.
When voltage is applied between the anode and the cathode, holes injected from the anode move to the emission layer through the hole transport layer and electrons injected from the cathode move to the emission layer through the electron transport layer. The holes and electrons which are carriers are recombined in the emission layer, thereby generating exitons. The exitons are transited from an excited state to a ground state, thereby generating light.
Light efficiency of organic light emitting diodes can be categorized into internal luminescent efficiency and external luminescent efficiency. Internal luminescent efficiency relates to how efficiently exitons are generated in an organic layer interposed between a first electrode and a second electrode, that is, between the anode and the cathode and light is converted, wherein the organic layer may be a hole transport layer, an emission layer, or an electron transport layer. External luminescent efficiency (hereinafter also referred to as light coupling efficiency) relates to how efficiently light generated in the organic layer is emitted outside the organic light emitting diode. Accordingly, even if the organic layer has high light conversion efficiency, that is, the internal luminescent efficiency is high, when the light coupling efficiency is low, the whole light efficiency of the organic light emitting diode may be decreased.